Recently, various attempts have been made to provide illumination systems emitting white light by using light-emitting diodes as radiation sources.
A first category of illumination systems emitting white light by using light-emitting diodes is based on the use of multiple visible light-emitting diodes. In these systems, two monochromatic LEDs (e.g. blue and yellow) or three monochromatic LEDs (e.g. red, blue and green) are used in combination. The light from the multiple visible light-emitting diodes is mixed to create a whitish light. However, when generating white light from an arrangement of monochromatic red, green and blue light-emitting diodes, there is such a problem that white light of the desired tone cannot be generated due to lifetime variations of the tone, luminance and other factors of the light-emitting diodes. Complex drive electronics are also necessary to compensate for the differential aging and color shifting of each LED.
In order to overcome these difficulties, illumination systems of a second category have been developed, which convert the color of a light-emitting diode by means of a luminescent material comprising a phosphor to provide visible, preferably white, light illumination.
Such phosphor-converted white light illumination systems are based in particular either on the trichromatic (RGB) approach, i.e. on mixing three colors, namely red, green and blue, in which case the components of the blue output light may be provided by a phosphor and/or by the primary emission of the LED, or in a second, simplified solution, on the dichromatic (BY) approach, mixing yellow and blue colors, in which case the yellow secondary component of the output light may be provided by a yellow phosphor, and the blue component may be provided by a phosphor or by the primary emission of a blue LED. The latter is the most common phosphor-converted system.
In particular, the dichromatic approach as disclosed in e.g. U.S. Pat. No. 5,998,925 uses a blue light-emitting diode of InGaN-based semiconductor material combined with a Y3Al5O12:Ce (YAG-Ce3+) garnet phosphor. The YAG-Ce3+ phosphor is coated on the InGaN LED, and a portion of the blue light emitted from the LED is converted to yellow light by the phosphor. Another portion of the blue light from the LED is transmitted through the phosphor. The combination of the yellow and the blue emission creates a convincing perception of whiteness with a typical CRI in the mid-70s and a color temperature Tc that may range from about 6000 K to about 8000 K.
A limitation of such phosphor-converted light-emitting devices stems from a low color rendition.
When illuminated by this type of phosphor-converted LED, an object does not appear natural to the human eye. Colors appear hyper-real or more vivid than under midday sunlight but also appear less differentiated from one another, as the ability of a white light source to accurately reveal colors depends on the number and intensity of the colors contained in the light coming from that source.
The figure of merit for true color rendition is the color-rendering index (CRI). The CRI is a relative scale ranging from 0 to 100, indicating how perceived colors match actual colors. It measures the degree by which perceived colors of objects, illuminated by a given light source, conform to the colors of the same objects when they are lit by a reference standard light source. The higher the color rendering index, the less color shift or distortion occurs.
In the typical solid-state lighting device, which is constituted by the combination of a blue LED and a yellow phosphor, the amount of light emitted therefrom in the red range (not less than 600 nm) and the bluish green range (480 to 510 nm) is notoriously small, and the light emitted from the white LED has a low general color rendering index as well as a low special color rendering index R5 that represents a color rendering property in the bluish green range.
Accordingly, in order to compensate for the deficiency in the red range of the dichromatic spectrum of white light created by these devices, research has been conducted on the control of phosphor characteristics, on the partial replacement of the host and/or activator with another element and on the use of mixed phosphors. FIG. 2 shows emission, excitation, and reflection spectra of the prior-art red-deficiency compensating phosphor YAG:Ce+Pr.
Nevertheless, the addition of red radiation to a blue-yellow spectrum for the purpose of “color correcting” or lowering the CCT (Correlated Color Temperature) of its white light emission often results in light that appears unnaturally pinkish and lowers the color contrast between adjacent objects or in printed images having different colors.